1. Field of the Invention
The present invention relates to driving and control of an image forming apparatus for forming an image on a recording medium.
2. Description of the Related Art
In a color image forming apparatus, output images are required to be high quality, and one of the items of the quality of output image includes misregistraton. In order to reduce this color deviation amount, the following processing is performed, for example: toner patches of respective colors are formed on an intermediate transfer belt to detect the color deviation amount, and the positions of the toner patches are detected by registration detection sensor; and the respective color images are written to a photosensitive drum based on the detection result.
An image forming apparatus, in which a plurality of image forming units including photosensitive drums are successively operated, is known to have misregistration that is caused by velocity variation of an intermediate transfer belt. When a transfer material conveying belt, i.e., the intermediate transfer belt, changes its velocity, force exerted on the belt by an image bearing member is changed at a transfer nip at each of the color image forming units. Accordingly, a pulling force or a pushing force of the belt occurs between the transfer nips of the color image forming units, and the belt passes the respective transfer nips at respectively different velocities, which causes misregistration. This is because, when the circumferential velocities of the photosensitive drum and the intermediate transfer belt are different, a frictional coefficient between the photosensitive drum and the intermediate transfer belt changes according to whether a toner enters into a primary transfer nip part, and accordingly a force in the tangent direction changes.
For example, Japanese Patent Application Laid-Open No. 2003-228217 suggests the following solution for this problem. In the technique of Japanese Patent Application Laid-Open No. 2003-228217, charging, developing, and transferring steps performed in image forming units, which are the cause of load variation, are turned on and off while a visible image is not transferred from a drum onto an intermediate transfer member, thus preventing the image from being affected by velocity variation of the intermediate transfer member.
The above technique of Japanese Patent Application Laid-Open No. 2003-228217 does solve the above problem. However, it takes a long time to perform the steps of charging and developing in this method for turning on and off the charging, developing, and transferring steps performed in image forming units while a visible image is not transferred from a photosensitive drum onto the intermediate transfer belt. As a result, there is a possibility that the lifespan of the image forming units may be excessively reduced.
In the first place, the above-described problem of misregistration can be alleviated by reducing the difference between the peripheral velocity of the photosensitive drum and the peripheral velocity of the intermediate transfer belt. In other words, a mechanism for alleviating the difference between the peripheral velocity of the image bearing member such as a photosensitive drum and the peripheral velocity of the intermediate transfer member such as an intermediate transfer belt is desired.
Further, the applicant has studied this issue, and has found that the misregistration caused by the velocity variation of the intermediate transfer belt is changed not only by the difference between the peripheral velocity of the image bearing member and the peripheral velocity of the intermediate transfer member but also by factors such as the state of use of the intermediate transfer belt. In addition, the above occurs not only by the intermediate transfer belt but also by the relationship between the image bearing member and the transfer material conveying belt in the same manner.
Therefore, a mechanism is desired to alleviate the color deviation amount by flexibly reducing the difference of the peripheral velocities between an image bearing member and a rotating member, such as an intermediate transfer member and a transfer material bearing member, that is arranged to face the image bearing member.